Giant magneto resistive (GMR) sensors were first manufactured in the 1980's. They are distinguished by their high sensitivity of their electrical resistance to the orientation of an external magnetic field. The GMR effect takes place in a limited range along one axis of the magnetic field. This range is referred to as the anisotropic range. In the anisotropic range, the sensor has a high sensitivity (resistance change versus magnetic field change). In some applications, such as an incremental speed sensor with a back bias magnet for measuring the speed of a magnetic tooth wheel, a small misplacement or inclination of the back bias magnet with respect to a GMR element of the sensor can drive the working point of the GMR element into saturation. The back bias magnet creates an offset in the working point of the sensor from an optimal point near the center of the anisotropic range, to the saturation region. As a result, no signal or a signal of reduced quality is generated, thereby lowering the sensor performance. For gradiometric sensors that operate on a differential principle, the offset problem becomes worse as the sensors are located in two different positions.
The problem of a GMR element being driven into saturation can be avoided by an increase of the anisotropic range. The anisotropic range where the sensor is sensitive can be extended with technology variations or geometric variations of the sensor, but then different sensors for different applications have to be developed. Furthermore, the extension of the sensitivity range by geometric variations (e.g., lowering the GMR structure width) is restricted due to limitations of state-of-the-art etching processes for the GMR layer.
One prior approach uses a groove or cavity in the magnet or a shaped metal plate to keep magneto resistive sensors in an optimal working point. Disadvantages of this approach are the higher costs for the additional grooving of the magnet or the additional special shaped metal plate.
Another solution for reducing the offset induced by the back bias magnet is to reduce the strength of the magnet. However, this approach reduces proportionally the magnetic signal generated by the passing magnetic tooth wheel.